1. Field of the Invention
The invention relates to a bottle-shaped trench fabrication method, and more particularly to a fabrication method for forming a bottle-shaped trench by plasma nitridation.
2. Description of the Related Art
Generally speaking, capacitors widely used in dynamic random access memory (DRAM) are formed by two conductive layers (electrode plates) having an insulation layer therebetween. The ability to store the electric charge of a capacitor depends on the thickness and electrical characteristics of the insulation layer, and the surface area of the electrode plate. Recent developments to reduce the size of semiconductor elements to enhance integration density require the memory cell area to be continuously reduced to hold a large number of memory cells, and thereby increase density. The electrode plates of a capacitor in a memory cell however must provide sufficient surface area to store sufficient electric charge.
Nevertheless, when element size is continuously reduced, the trench storage node capacitance of DRAM is also reduced. As a result, storage capacitance must be increased to maintain excellent memory operating performance.
Currently, the method for increasing DRAM storage capacitance increases the width of the bottom of a trench, thereby increasing surface area by forming bottle-shaped trench capacitors.
FIGS. 1A to 1K are cross-sections showing the conventional method for forming a bottle-shaped trench. In FIG. 1A, a substrate 100 with a trench therein is provided, a pad layer is formed on the substrate 100, and the pad layer comprises an oxide layer 102 and a nitride layer 104. A sidewall oxide layer 106 is formed on the trench sidewall by thermal oxidation. A protective nitride layer 108 and a polysilicon layer 110 are formed thereon by chemical vapor deposition (CVD). In FIG. 1B, the polysilicon layer 110 is oxidized forming a protective oxide layer 120. In FIG. 1C, a mask layer 122 is formed to cover the lower portion of the trench, the mask layer 122 comprises, for example, a photoresist material. In FIG. 1C, the protective oxide layer 120 not protected by the mask layer 122, is removed to form a protective oxide layer 120′. Then the mask layer 122 is removed as in FIG. 1E. In FIG. 1F, a sidewall nitride layer 124 is formed on the upper portion of the trench by nitridation. The protective oxide layer 120′ is then removed as shown in FIG. 1G. In FIG. 1H, the protective nitride layer 108 is then removed to form a protective oxide layer 108′. The sidewall nitride layer 124 is removed as shown in FIG. 1I. In FIG. 1J, the lower portion of sidewall oxide layer 106 is removed to form a sidewall oxide layer 106′. In FIG. 1K, the lower portion of the trench is etched to form a bottle-shaped trench by wet etching.
The above-mentioned bottle-shaped trench fabrication method is complicated, and due to the continuous reduction in semiconductor scaling, this fabrication method is progressively more complicated. Therefore, a simple fabrication method for forming the bottle-shaped trench is necessary.